Single-pen, black or tri-color ink-jet printers are well-known. If it is desired with a tri-color ink-jet printer to produce black, color dots are closely interposed while printing, generally in equal spatial proportion, to give a black appearance. Such a black appearance produced by closely interposed tri-color ink dots (as well as gray-scaled output produced by variably less closely interposed tri-color ink dots) will be referred to herein as composite or process black, as opposed to what will be referred to herein as true black produced by depositing black ink drops on paper. Because a given one of these ink-jet printers might be equipped with either a true black pen or a tri- color pen, the host computer or file server, or what will be referred to herein as the driver software operating therein, somehow must be able to decide when to use process black and when to use true black. Black-to-color bleed that might occur in printed areas containing closely adjacent true black and color ink dots would require the driver software or the printer firmware, or both, to determine and to control when a black ink pen is to be used to produce true black and when process black is to be used.
The black-to-color bleed problem to which certain low-cost ink- jet printers are susceptible principally is caused by fundamental incompatibilities between the black ink and any of the tri-color inks. Black ink selection is somewhat constrained by requirements such as water-fastness and compatibility with regular paper stock. Accordingly, there are many ink-jet printer applications in which the use of incompatible black and color inks will continue. As a result, black-to-color bleeding problems remain.
The black-to-color bleed problem previously has been addressed by software methods that determine when to use true black and when to use process black for various printing applications. One such method is described in U.S. patent application Ser. No. 07/784,498 entitled "Color Separation in Ink-Jet Color Graphics Printing", which was filed Oct. 29, 1991, issued as U.S. Pat. No. 5,168,552 on Dec. 1, 1992, and which is subject to common ownership herewith. Familiarity with the algorithm described therein is assumed. In practice, implementation of the algorithm requires that the printer's firmware be able to determine when a black print control code received from the application software is to be printed with true black (K) and when it is to be printed with process black. Briefly, such determination might involve examining received tri-color dot placement data as to whether more than one color of ink, whether the primitives including cyan, magenta and yellow (CMY) or the primaries including red, green and blue (RGB), is destined for placement on the same pixel or dot location on the printed page. If so, the two-pen printer then could substitute each one of such interposed color dot occurrences with a single true black dot.
With the advent of two-pen ink-jet printers, and depending upon the printing application, it may be desirable to print either or both of true black and process black using a two-pen ink-jet printer that includes both a black pen and a tri-color pen, which the above-described automatic substitution method would render impossible. Accordingly, it is desirable to develop a full-color palette selection and control system for more capable ink-jet printers that is nevertheless compatible with existing software drivers and black-and-white and color printing applications. Preferably, the system would permit printing either process black using a tri-color ink-jet pen or true black using a black ink-jet pen, or both, and would be organized to be compatible with existing products and to be efficient in terms of color control coding and selection so that unneeded data not be sent to the printer.